Investigation of ferromagnetic domain behavior and phase transition at nanoscale in bilayer manganites

TL;DR

This study investigates the nanoscale ferromagnetic domain behavior and phase transition in bilayer manganites using Lorentz transmission electron microscopy, revealing new insights into magnetic nanoclusters near the Curie temperature.

Contribution

It provides the first observation of ferromagnetic nanoclusters in bilayer manganites and analyzes the influence of magnetocrystalline anisotropy on domain wall structure.

Findings

Magnetic domains appear at the Curie temperature within the a-b plane.

The ferromagnetic phase transition involves nanoscale granular contrast.

Presence of ferromagnetic nanoclusters near the Curie temperature.

Abstract

Understanding the underlying mechanism and phenomenology of colossal magnetoresistance in manganites has largely focused on atomic and nanoscale physics such as double exchange, phase separation, and charge order. Here we consider a more macroscopic view of manganite materials physics, reporting on the ferromagnetic domain behavior in a bilayer manganite sample with a nominal composition of La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ with $x=0.38$, studied using in-situ Lorentz transmission electron microscopy. The role of magnetocrystalline anisotropy on the structure of domain walls was elucidated. On cooling, magnetic domain contrast was seen to appear first at the Curie temperature within the $a-b$ plane. With further reduction in temperature, the change in area fraction of magnetic domains was used to estimate the critical exponent describing the ferromagntic phase transition. The ferromagnetic phase transition was accompanied by a distinctive nanoscale granular contrast close to the Curie temperature, which we infer to be related to the presence of ferromagnetic nanoclusters in a paramagnetic matrix, which has not yet been reported in bilayer manganites.